1. Field of the Invention
The invention is concerned with the provision of fixation devices that improve tissue repair and with the use of such devices in medical treatment. In particular, the invention is concerned with the fixation of tissues, which have limited or no vascular supply, and where angiogenesis is desirable or a prerequisite for good tissue repair.
2. Related Art
A wide variety of fixation devices exist for use in invasive medical treatments. These devices may be used to rejoin, re-affix, hold, or otherwise partake in the repair of tissue during and after surgery and other medical treatments.
An aim of medical practitioners following surgery, for example, is to incite rapid healing and tissue repair throughout the treatment site. A factor in the promotion of tissue repair is the extent to which reparative cells and other factors can permeate through to the tissue in question. This, in turn, is dependent upon the extent to which blood vessels can form in and around the site.
The formation of new blood vessels from pre-existing ones is known as angiogenesis. Angiogenesis is an essential process during development of the human body, particularly embryonic development. Development of the human embryo commences with fusion of blood islands into vascular structures in the process of vasculogenesis; subsequently angiogenesis begins, with new vessels sprouting off from the vessels formed during vasculogenesis. Angiogenesis normally tails off, however, when the body becomes adult. With the exception of the female reproductive system, angiogenesis in the adult mainly occurs during tissue repair after wounding or inflammation, although it is also associated with adult pathological conditions, such as tumor growth, rheumatoid arthritis, psoriasis, and diabetes.
The principle cell type involved in angiogenesis is the microvascular endothelial cell. Following injury and/or in response to angiogenic factors, the basement membrane of endothelial cells in the parent blood vessel is degraded, a process mediated by endothelial cell proteases. Once the basement membrane is degraded, endothelial cells migrate out into the perivascular space. Cells at the base of the sprout proliferate and replace the migrated cells. A new basement membrane is then formed and two contiguous sprouts fuse together to form a loop. Subsequently a lumen forms and blood begins to flow.
The endothelial cell is the central cell type involved in angiogenesis because it is capable of expressing all the necessary information for the formation of new microvascular networks. It appears to achieve this by acting in concert with many different cell types to form new vessels. While not wishing to be bound by any theory, these other cell types may promote angiogenesis by expressing growth factors and cytokines that stimulate the proliferation and migration of the primary cellular components of the vascular wall, including endothelial cells.
Therapeutic angiogenesis is the clinical use of angiogenic factors or, in some cases, genes encoding these factors to enhance or promote the development of blood vessels in ischaemic/avascular tissue. The ideal agent for therapeutic angiogenesis would be safe, effective, inexpensive, and easy to administer. It would also be highly desirable to provide angiogenic factors in a controlled way over a predetermined period of time.
It is an aim of the present invention to provide medical devices, which release angiogenic factors that promote blood vessel formation in the surrounding tissue.
It is a further aim of the present invention to provide fixation devices which promote tissue repair in the surrounding tissue and which will release factors that promote blood vessel vaso-dilation. Delivery of such factors or pharmacological active agents using fixation devices, for example a suture or surgical tape, requires the suture or surgical tape to have certain characteristics to enable it to function as both a suture or surgical tape and a fixation or delivery device. For instance, the suture must have adequate strength, knot tying and sliding properties for it to function in its primary role, as a suture. As a biological agent delivery device, the suture must be able to incorporate the active agent within its structure in such a way that the active agent is not changed in any way during the loading. The active agent must be stable within the structure to allow for normal storage prior to use. Once in place in vivo the active agent must be released at a certain dose over a certain time period to maximise its therapeutic properties and minimise adverse reactions. Suture repair, using a standard suture, of certain tissues which have limited or no vascular supply, for example, meniscal cartilage, articular cartilage, ligaments, tendons, bone, and ischaemic tissue can be problematic.